Conventional air-cure alkyd coatings utilize, as the binder or film-forming component, a curing or drying alkyd resin. By the term "alkyd coating", as used hereinafter, is meant a conventional liquid coating based on alkyd resins, typically a paint, clear coating, or stain. The alkyd resins are complex branched and cross-linked polyesters containing unsaturated aliphatic acid residues. The methods for preparing alkyd resins and formulating alkyd coatings from them are well known and are described in Surface Coatings Vol. I, Raw Materials and Their Usage (Chapman and Hall, New York N.Y., Second Edition, 1984).
Drying oils are liquid vegetable or synthetic oils such as linseed oil, soybean oil, safflower oil, and dehydrated castor oil. Linseed oil, a typical example of drying oils, is obtained from seeds of the common flax plant (Linum usitatissimum) and contains, inter alia, esters of glycerol with such unsaturated aliphatic acids as linolenic acid, linoleic acid and oleic acid.
Drying oils, and alkyd resins containing the unsaturated aliphatic acids derived from drying oils, spontaneously polymerize in the presence of air to yield a solid protective film. The polymerization is termed "drying" or "curing" and occurs as a result of autoxidation of the unsaturated carbon-carbon bonds in the aliphatic acid component of the oil by atmospheric oxygen. When applied to a surface as a thin liquid layer of formulated alkyd coating, the cured films that form are relatively hard, non-melting, and substantially insoluble in many organic solvents that act as solvents or thinners for the unoxidized alkyd resin or drying oil. Hereinafter the term "cured" is used to describe the autoxidized dried coating. Such drying oils have been used for centuries as raw materials for oil-based coatings and are described widely in the literature, e.g., in Surface Coatings Vol. I.
Conventional alkyd coatings use a prepolymer such as a drying oil alkyd resin as the film forming component in the cured alkyd coating. While these hydrocarbon polymers can give reasonable water repellency, they have little or no oil repellency. This lack of oil repellency can lead to the problem of staining by oily stains and are thus susceptible to soiling. In particular they have no resistance to wetting by oils, as demonstrated by their very low hexadecane contact angles.
Another conventional coating is the Type I urethane coating. Urethane coatings are classified by ASTM D-1 into five categories. Type I urethane coatings contain a pre-reacted autoxidizable binder as described in Surface Coatings Vol. I, previously cited. Type I urethane resins (binders), also termed urethane oils, oil-modified polyurethanes, or urethane alkyds, are the largest volume category of polyurethane coatings. By the term "urethane coating", as used hereinafter, is meant a conventional liquid coating based on Type I urethane resins, typically a paint, clear coating, or stain. Urethane coatings typically contain the reaction product of a polyisocyanate, usually toluene diisocyanate, and a polyhydric alcohol ester of drying oil acids. The cured coating is formed by air oxidation and polymerization of the unsaturated drying oil residue in the binder.
Urethane coatings give cured coatings with many desirable properties, but the cured coatings have little or no oil repellency as demonstrated by their very low hexadecane contact angles.
Certain commercial materials are known which provide oil repellency to textile and carpet substrates, such as aqueous perfluoroalkyl emulsion polymers (Fasick et al., U.S. Pat. No. 3,378,609, Dettre et al., U.S. Pat. No.3,923,715, Raynolds et al., U.S. Pat. No. 3,462,296, U.S. Pat. No. 3,491,169 and U.S. Pat. No. 4,595,518, and Pechhold, U.S. Pat. No. 4,958,039). The treated substrates have outstanding oil and water repellency, however, the emulsion fluoropolymers utilized are not compatible with alkyds and urethanes.
The use of fluoroalkyl alcohol esters of alkanoic acids generally as lubricating aids is well known. For instance, the perfluoroalkyl ethyl ester of stearic acid has been used for imparting lubricity and repellency to various plastics. Also, Nishihara et al., JP308469 (1989), disclose the preparation of aliphatic carboxylic acid esters of various fluorinated alcohols, which can contain double bonds in the acid portion, and their use as lubricants for ferromagnetic metal thin film-type magnetic recording media.
Adding perfluoroalkyl ethyl stearate, a non-curing ("non-drying") fluoroalkyl ethanol ester of a saturated vegetable oil, to alkyd or urethane coatings in suitable formulations, however, provides at best only temporary oil and water repellency because the fluorinated component migrates to the surface but is not chemically bound with the autoxidized polymer. Thus the oil repellency is not durable and is readily lost when the surface is washed or otherwise cleaned.
It would be highly desirable to be able to provide cost effective, durable oil repellency to drying oil, alkyd, and urethane coatings.
There is a need to have a fluorinated moiety present at the liquid/air interface after application of the coating and prior to curing that is chemically bound, such as by crosslinking, to the coating to provide durable surface oil repellency.